With 8 people plus luggage, I'd really recommend considering a 15 passenger van. That is a long drive, and a van will enable you to stretch out a bit more and also likely be considerably cheaper.

Explore all the agencies. Enterprise, for example, has a cheaper underage fee, but they might not allow you to drive to Colorado -- you should check. Finally, if you have any flexibility, renting not from the airport will save quite a bit of money as well.